Global Dimming

Global Dimming

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in 1950s.

The effect varied by location, but worldwide it was of the order of a 4% reduction over the three decades from 19601990. The trend reversed during the past decade. Global dimming has interfered with the hydrological cycle by reducing evaporation and may have caused droughts in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.

Cause and effects

It is thought that global dimming was probably due to the increased presence of aerosol particles in the atmosphere caused by human action. Aerosols and other particulates absorb solar energy and reflect sunlight back into space. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets).

The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the earth's surface. Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the Earth's heat loss. Also it has been thought that humans help produce the particles in the earth's atmosphere, resulting in global dimming.

Research

In the late-1960s, Mikhail Ivanovich Budyko worked with simple two-dimensional energy-balance climate models to investigate the reflectivity of ice. He found that the ice-albedo feedback created a positive feedback loop in the Earth's climate system. The more snow and ice, the more solar radiation is reflected back into space and hence the colder Earth grows and the more it snows.

Other studies found that pollution or a volcano eruption could snap us into an ice age. In the mid-1980s, Atsumu Ohmura, a geography researcher at the Swiss Federal Institute of Technology, found something that left him in a shock. He found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades.

His findings are in apparent contradiction to global warmingthe global temperature has steadily been going up. Less light reaching the earth would mean that it would have to cool. Ohmura published his findings "Secular variation of global radiation in Europe" in 1989.

This was soon followed by others. Vivii Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia", and Beate Liepert in 1994 "Solar radiation in Germany - Observed trends and an assessment of their causes". Dimming has also been observed in sites all over the Former Soviet Union. Gerry Stanhill who studied these declines worldwide in many papers (see references) coined the term "Global dimming".Independent research in Israel and the Netherlands in the late 1980s showed an apparent reduction in the amount of sunlight, despite widespread evidence that the climate was actually becoming hotter. The rate of dimming varies around the world but is on average estimated at around 23% per decade, with the possibility that the trend reversed in the early 1990s. It is difficult to make a precise measurement, due to the difficulty in accurately calibrating the instruments used, and the problem of spatial coverage. Nonetheless, the effect is almost certainly present.The effect (23%, as above) is due to changes within the Earth's atmosphere; the value of the solar radiation at the top of the atmosphere has not changed by more than a fraction of this amount.

Pan evaporation data

Over the last 50 or so years, pan evaporation has been carefully monitored. For decades, nobody took much notice of the pan evaporation measurements. But in the 1990s scientists spotted something that at the time was considered very strange, the rate of evaporation was falling although they had expected it to increase due to Global warming. The same trend has been observed in China over a similar period.

A decrease in solar irradiance is cited to be the driving force. However, unlike in other areas of the world, in China the decrease in solar irradiance was not always accompanied by an increase in cloud cover and precipitation. It is believed that aerosols may play a critical role in the decrease of solar irradiance in China.

BBC Horizon producer David Sington believes that many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming. Pan evaporation experiments are easy to reproduce with low-cost equipment, there are many pans used for agriculture all over the world and in many instances, and the data has been collected for nearly a half century.

However, pan evaporation depends on some additional factors besides net radiation from the sun. The other two major factors are vapor pressure deficit and wind speed. The ambient temperature turns out to be a negligible factor. The pan evaporation data corroborates the data gathered by radiometer and fills in the gaps in the data obtained using pyranometers. With adjustments to these factors, pan evaporation data has been compared to results of climate simulations.